Can milk protein percentage serve as a genetic indicator of fertility (calving interval) in dairy cattle?

Data on milk yield traits, survival from first to second lactation and calving interval (CI) were analysed to estimate the relationships among them and determine whether the reliability of estimated breeding values (BVs) for CI could be increased from multi-trait analyses. The data consisted of 87,942 first parity cows which were daughters of 1679 sires. The genetic correlations of milk, protein and fat yield with CI were unfavourable (0.4 to 0.5) and were not influenced by stage of lactation while environmental correlations were slightly higher (~0.05) towards the end of the lactation (181 to 210 days) than at the beginning of the lactation (6 to 91 days). Protein percentage was favourably correlated both genetically (at the beginning) and environmentally (throughout the lactation) with CI. However, the genetic correlation between CI and protein % was slightly weaker than between CI and other milk yield traits. Consequently, protein % did not improve the reliability of estimated BVs for CI whereas milk yield resulted in a modest increase